Telephone-exchange system



July 20 1926.

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E. H. CLARK TELEPHONE EXCHANGE SYSTEM File ly 5, 1924 8 Sheets-Sheet 2 by M, 141732 July 20, 1926. 1,593,387

E. H. CLARK TELEPHONE EXCHANGE SYSTEM Filed July 5. 1924 8 Sheets-Sheet 5 July 20 ,-1926.

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- E. H. CLARK TELEPHONE EXCHANGE SYSTEM Fil y 5, 1924 8 Sheets-Sheet 6 "/flvebfal': fdyar x6. K/a/K i by My July 20 1926. 1,593,387

E. H. CLARK TELEPHONE EXCHANGE SYSTEM Fil y 5. 1924 8 Sheets-Sheet 7 bv M July 20, 1926. 1,593,387

E. H. CLARK TELEPHONE EXCHANGE SYSTEM Filed July 5, 1924 8 Sheets-Sheet 8 Patented July 20, 1926.

UNITED STATES PATENT OFFICE.

EDGAR H. CLARK OF RICHMOND HILL, NEW YORK, ASSIGNOR TO WESTERN ELEC- TRIO COMPANY, INCORPORATED, 013 NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TELEPHONE- EXCHANGE SYSTEM.

Application filed July 5, 1924. Serial No. 724,363.

. exchanges.

Accordingly, it is an object of the invention to improve the method of testing large groups of trunks.

A feature of the invention relates to a testing mechanism which tests a selected group of trunks by sections, the connection If so to, and testing-of, a particular section, being dependent upon the condition of the trunks in the preceding section.

A further feature of the invention relate to a connecting'device which by series of successive operations serves to associate a limited number of test relays with a greater number of trunks.

Other features and advantages of the invention will become apparent from a consideration of the following detailed description and from the appended claims.

The several figuresof the drawing when arranged in the order illustrated in Fig. 9 disclose in suflicient detail a preferred form of carrying out the invention.

Fig. 1 shows in schematic form a line switch, district selector switch, and sender selector switch, all of the coordinate type, together with one of a number of calling lines, and interconnecting trunks.

Fig. 2 in the upper central portion shows aschematic representation of an incoming selector switch. To the right of this figure is shown a trunk connecting device, while in the lower portion is shown the marker testing mechanism.

Fig. 3 shows a final selector switch and a schematic diagram of a private branch exchange.

Fig. 4 shows a connecting device located in the marker for .associating trunks leading to the private branch exchange with certain marker testing equipment.

Fig. 5 to the left illustrates diagrammatically a subscribers registering and controltesting groups of trunks leading toa private branch exchange.

Detailed description. The line switch 100, sender selector switch 160, district switch 150 (represented schematically), incoming switch 200 and final switch 300 are all of the coordinate type, such as disclosed in the application of C. L. Goodrum and J. N. Reynolds Serial No. 486,442, filed July 21, 1921, issued as Patent No. 1,515,735 November 18, 1924, and in the application of S. B. Williams, Serial No. 596,179, filed October 23, 1922, issued as Patent No. 1,517,331 December 2, 1924, to which reference may be had for a detailed description of the construction and opera-.

tion of these switches.

Inasmuch as the present invention is mainly concerned with the means for completing connections to private branch exchanges, the selective'operation of the line switch, district switch, and incoming switch together with the controlling equipment provided therefor, will be referred to in a general way.

Let it be assumed that the subscriber at station 101 has initiated a call with the purpose of conversing with a subscriber whose line is located in the private branch exchange 350. The calling line 102 is extended by means of switches 100 and 150 to the incoming trunk 251. At the same time the sender selector 160 operates to connect the calling linewith the recording and controlling sender 500. The sender 500 is also associated by means of connector 560 with an incoming marker registering and controlling mechanism. The numerical portion is connected to conductor 651 and thence through the right winding of relay 701 to ground at the armature of relay 7 For a detailed description of the manner in wh ch the toregoingoperations are effected, reterence may be had to the application of V, f. Carpenter, Serial No. 703,362 filed April 1, 1024. Relay 701 closes a locking circuit for itself in series with the winding of relay 702 which operates. lVith relays 701 and 702 operated, ground is connected through the outer armature of relay 701, thence in parallel through the windings of relays 703, 704 and 202 and these relays operate. This same ground is also extended through the right inner armature and contact of relay 701, over conductor 653 to battery through the winding of relay 615 which immediately locks to the grounded conductor 652. [u the manner described in detail in the abovementioned application of W'. W. Carpenter, Serial No. 703,362, filed April 1, 1924, relay 615 brings'about the selective operation of incoming frame connector 230 which in turn causes the operation of vertical magnet 215 as well as the proper trunk group relay as 208.

When relay 703 operated it connected the various conductors from the incoming marker through to the final translator shown in Fig. 7. In the manner described in detail in the above1nentioned application of 1V. 1V. Carpenter, the setting of the hundreds register 510 causes ground to be extended over one of the conductors 550, 551, etc. Assuming conductor 551 to be thus grounded, a circuit is completed through the contacts of relay 703 to operate relay 705. Assuming that the units digit of the P. B. X. call line designation is 6, then as described in the above-mentioned application of 1V. W. Carpenter, battery is applied to conductor 552 causing the operation of relay 706. In a similar manner, the setting of the tens register 520 causes one of the conductors 553, 554, etc, to be grounded. Assuming conductor 554 to be thus grounded, a circuit is completed for vertical magnet: 310 associated with vertical row of contacts 315 in which the P. B. X. call line is located. The circuit for operating magnet 310 may be traced from battery through its winding. conductor 352, contacts of relays 706 and 703 to the grounded conductor 554. The setting of units register 515 now causes ground to be extended by means of conductor 652, armature and back contact of relay 622 inner armature and back :ontact of relay 625, conductor 650, thence to conductor 555 through contact of relay 703. to battery through the winding of relay 710 which operates. Relay 710 is individual to section 325 in switch 300. Similarly, relay 707 is individual to section 328.

hen magnet 310 operated it extended ground through its lower armature and contact over conductor 353 to operate multicontact relay 708. llith relay S operated. a circuit is completed for the horizontal test magnet 331. This circuit may be traced from battery through the right. winding of magnet 331, conductor 354, contacts of relays 708, 710 and 704. conductor 751 and thence to the grounded conductor (152. .is soon as magnet 331 operates ground is extended by means of the upper conductor of the test link 327 and a contact in vertical row 315, through the connecting block 322 to operate relay 321. Relay 321 immediately locks through it innermost armature and contact, conductor 355, contact of relay 704 to the grounded conductor 751. Relay 321 extends this same ground through its middle armature and contact by means of conductor 356, through contacts of relay 704, conductor 752, to battery through the winding of relay 625. Relay 625 operates relays 401 and 711 in parallel circuits traceable through their windings, thence through contacts of relay 704, conductor 556 to ground through contacts of relay 625. Relay 401 connects the test relays 800 to the grid wires of the connecting device in Fig. 4 and relay 711 connects said test relays to the relays 707, 710, etc. Relays 625 in operating removes ground 11'0111 conductor 650 causing the release of relay 710. The removal of ground from conductor 650 also causes the release of vertical magnet 310 and the release of relay 710 causes the release of test horizontal magnet 331.

It will 'be observed that the group of trunks (of which four are shown), leading to the private branch exchange 350 are arbitrarily divided so as to appear before different vertical rows of contacts. Accordingly, it will be necessary not only to determine an idle trunk in this group but also to identify the particular vertical row of contacts in which this idle trunk appears. This is accomplished by means of the testing mechanism shown in Figs. 4 and 8 as will be described in detail hereinafter.

For the purpose of description, it will be assumed for the present that the first trunk 317 is busy and that the second trunk 318 is idle. lVhen relay 321 operated, it completed a circuit for operating multicontact relay 402. Relay 402 is individual to the first section of the group of trunks leading to the private branch exchange 350. Similarly, relays 403 and 404 are individual to other succeeding sections of this same trunk group. Still other relays may be provided each individual to a different section of other groups of P. B. X. trunks appearing in connector switches control ed by the same final translator. One of these other relays 467 is shown, Relay 467 is individual to the first section of a second group of P. B. X. trunks and functions similarly to relay 402. The circuit for operating relay 402 may be traced from ground through its winding, conductor 455, outer armature and contact of relay 321, conduct-or 3 59, contact of relay 401, conductor 456,

i outer armature and contact of relay 815 to battery through the left winding of relay 816. Relay 816 operates in this circuit, and looks through its right winding and locking contact, conductors 850 and 355 tions at block 421. Relays 818, 821, 822 and 823, are provided to prevent the transmission to the marker of an all-trunks busy indication until the trunks in all sections have been tested as will be explained. Suliicient test relays 800 are provided to accommodate the maximum number of trunkscontrolled by any one of the multicontact relays 402,403, 404. Assuming that the largest number of trunks is controlled by relay 402, it will be necessary, therefore, to associate all the test relays with this section of the trunk group. Accordingly, ground .isextended through contact-s of relays402 and 401 to battery through the windings ofthe respectiverelays 818, 821, 822, 823. Since the first trunk 317 has been assumed to be busy then relay 801 is operated in a circuit traceable from battery through its lower winding, conductor 852,

cont-acts of relays 401 and 402, conductor 463, to the grounded sleeve conductor of trunk 317. Relay 801 locks in a circuit traceable through its upper winding and locking contact, conductor 851, contactsof relays 401 and 402 and returning thence to the grounded conductor 850. Since the second trunk 318 is idle, relay 802 remains normal and when relay 817 finally closes its contacts a circuit is completedfrom battery' through the winding of vertical magnet311, conductor 351, thence by means of connecting blocks 452 and 453 through contacts of'relays 402 and 401. conductor 451, lower outer armature and back contact of relay 802, lower outer armature and front contact of relay 801, left armature and front contact of relay 818, armature and contact of relay 817, right armature and front contact of relay 818 to ground over conductor- 851 as above traced.

An idle trunk leading to the private branch exchange having been located and the proper vertical magnet operated it becomes necessary to associate the marker testing equipment 250, .with the links in switch 200, the links in section 325 of the final'switch, and with the group of trunks extending between switches 200 and 300 giving access to trunk 318. trunk 318 is in-the section 325 of the final switch, it will be necessary to reoperate' relay 710 to associate the group of links in this section with the marker testing mechamsm. Relay 710 is operated in a circuit traceable from battery through its winding, innermost armature and contact of relay 711, conductor 753, through the distributing block 825, lower inner armature and back contact of relay 802, lower inner armature and front contact of relay 801, left armature and front contact of relay 818, armature and contact of relay 817, right armature and front contact of relay 818 to the grounded conductor 851. The operation of relay 710 extends the grounded conductor 751 through' contacts of relays 704, 710 and 708, over conductor 354, to reoperate test magnet 331'. The links of switch 200,

Since the idle the' trunks in the group including trunk 226, as well as the links in setion 325 ofthe final switch, are now associated with the testing mechanism 250. When an idle link in each of these switches as well as an idle trunk as 226 has been selected, as described in detail in the above mentioned application of W. W. Carpenter, vertical magnets 217 and 326, and horizontal magnet 216 corresponding to the selected idle link in switch 200, as well as horizontal magnet 324 corresponding to the selected idle link in switch 300 are operated and the trunk 318 is seized.

When the connection has thus been completed through switches 200 and 300, as described in the above mentioned application of W. W. Carpenter, Serial No. 703,362, filed April '1, 1924, the grounded sleeve conductor of trunk 251 is extended through contacts of the said switches and thence by means of the lower conductor of test link 327,; over conductor 357, contacts of relay 708, conductor 358, through contacts of relay 202, conductor 253 to battery through the left winding of relay 632 which locks through its right winding to the grounded conductor 652. The operation of relay 632 at this time serves as an indication to the marker that an idle trunk 318 has been seized, and in the manner described in detail in the above mentioned application of W. W. Carpenter, the sender 500, connector 560 and the marker register and controlling mechanisms are released, thus releasing the operated vertical magnets in switches 200 and 300. The connection through these switches is now maintained by the continued energization of magnets 216 and 324.

Returning to the point where relay 402 operated and a test was made ofthe trunks controlled thereby, it will be assumed that all the trunks in this section are busy. Ac-

cordingl all the test relays 800 are operated ancl the grounded conductor 851 is extended through the right armature and front contact of relay 818, armature and contact of relay 817, left armature and front contact of relay 818, lower inner armatures and front contacts of relays 801, 802, 803, in series, armature and front contact of re- .lay 821, lower inner armatures and front contacts of relays 804, 805, 806, armature and front contact of relay 822, lower inner armatures and front contacts of relays 807, 808, etc., armature and front contact of relay 823, conductor 854, contacts of relays 401 and 402 to battery through the winding of relay 403. Relay 403 operates and brings about the release of relay 402 by means of the ground extendedthrough the fifth lowest contact of relay 403, contact of relay 401, conductor 457, through the left wind-..

ing of relay 816 to battery, thus shunting the winding of relay 402 which releases. It will be observed that the operation of relay 403 extends ground through its eleventh lowest contact, resistance 416, conductor 458 to battery through the left winding of relay 815. Relay 815 does not operate in this circuit until relay 402 is released, since the left winding of relay 815 is shunted by the battery connected through the tenth lowest contact of relay 402. Upon the release of relay 402, this shunt is reriioved and relay 815 operates, completing a locking circuit for itself through its right winding to the grounded conductor'850. Relay 815 also closes an obviouscircuit for slow operating relay 819. When relay 402 released it opened the locking circuits of the test relays which were operated corresponding to busy trunks in the first section of the trunk group. Since relay 403 operates prior to the release of relay 402 the test relays are immediately re-connected tothe sleeve conductors of the second section of the trunk group and only those test relays corresponding to idle trunks in this second section remain normal. The relay 819 being slow to operate does not attract its armature to close a holding circuit for the relays 800 until all such relays which correspond to idle trunks in the second section have had time to release, it they are in an energized condition. The new holding circuit may be traced through contacts of relay 403 to conductor 459. In a manner similar to that already described in detail in connection with the first section of the trunk group, an idle trunk in this second section is selected, the proper vertical magnet operated, and switches 200 and 300 operated to seize this idle trunk.

Assum ng that all the trunks in this second section are busy, then all the test relays connected thereto are operated. Ground is now extended through the armature of relay 819, conductor 459, contacts of relays 401 and 403, returning through contacts of relays 403 and 401, conductor 851, right armature and front contact of relay 818, armature and contact of relay 817, left armature and front contact of relay 818, and thence as above traced through the lower inner armatures and front contacts of the operated test relays to conductor 854, through contacts of relays 401 and 403 to batter through the winding of relay 404 whicli operates. Relay 404 immediately locks to the grounded conductor 850. .Relay 404 at its fourth lowest contact connects battery by means of .loop 420 and contacts of relays 401 and 403 toshunt the winding of relay 403 which releases. With relay 404 operated a circuit is completed from battery through the winding of relay 825, conductor 461, resistance 426, through contacts of relays 401 and 404to ground. Relay 825 is not energized i n'this circuit, however, until relay 403releases and disconnects the battery from its twelfth lowest contacts thus removing the shunt from the windingof relay 825. lVith relay 404 operated the test relays 800 are associated with the third section of the trunk group, and in the manner already described in detail an idle trunk is selected and connection completed thereto, through switches 200 and 300.

Should the number of trunks in the second section be less than the total number of test relays, for example 7, then the lower windings of the remaining relays 808, etc., instead of being connected to sleeve conductors of trunks, are connected by means of contacts of relays 401 and 403, to ground through connecting block 425. Thus all the test relays are in an operated condition, and ground is extended over conductor 854 to operate relay 404 as already described.

Assuming that the third section of the trunk group consists of but four trunks and that all these trunks are found busy. Accordingly relays 801 to 804 inclusive which are connected to the sleeve conductors of this last section of the trunk group operate and lock to the grounded conductor 851. It will be noticed that by means of the cross connecting block 424 ground may be extended through the windings of all the remaining test relays 805, 806, 807, 808, etc., which operate, and lock to the grounded conductor 851. Accordingly, all the test relays are operated. Under this condition, ground is extended through the armature and contact of relay 825, conductor 462, through contacts of relays 401 and 404, returning thence by means of conductor 851, through the right armature and contact of relay 818, armature and contact of relay 817 left armature and front contact of relay 818, lower inner armatures and front contacts of relay 801, 802, and 803, armature and front contact of relay 821, lower inner armatdres and front contacts of relays 804, 805'and 806,

armature and front contact of relay 822, lower inner armatures and front contacts of the trunk group.

It will now be assumed that the: trunks controlled by relay 404 constitute the last section of the selected trunk group and that this section includes but four trunks. In this instance, conductors 471 and 472 will be .disconnected and when relay 403 releases, it

wiil bring about the release of relays 822 and 823, relay 821 remaining operated. Should therefore, all the trunks in this final section be busy, thetest relays 801 to 804 inclusive which are connected thereto operate and lock to the grounded conductor 851, and relays 805 and 806 also operate to ground at block 424. Under this condition,

ground is extended through the armature of relay 825, conductor 462, contacts of relays 401 and 404 returning thence by means of conductor-851 through the right armature and front contact of relay 818, armature and contact of relay 817, left armature and front contact of relay 818, lower inner armature and front contacts of relays 801, 802 and 803, armature and front contact of relay 821, lower in'ner armatures and front contacts of relays 804, 805 and 806, armature and back contact of relay 822, conductor 856, conductor 358, contacts of relay 202, conductor 253 to battery through the left winding of relay 632. Relay 632 immediately looks through this right winding to the ground of conductor 652. In the manner described in detail in the above mentioned application of W. W. Carpenter, the operation of rela 632 at this time serves as a line busy in ication to the marker which releases in turn releasing the testing equipment, the operated vertical magnets in the switches 200 and 300 and causes a characteristic busy tone to be transmitted to the calling subscriber. It will be seen therefore that the relays 821, 822, '823 control the transmission of a busy signal to the calling subscriber in accordance with the size of the last portion of the trunk group tested. If the last portion had consisted of but three trunks then relays 801, 802, 803, would, be operated because of the busy condition of these trunks, while the remaining test relays would be normal. The conductors 471, 4.7.2

and 473 would be disconnected and rela s 821, 822 and 823 in this instance would e normal and ground is extended'through'the lower inner armatures and front contacts of relays 801, 802, -803, back contact of relay 821, conductor 856, to operate busy test relay 632 as already'descnbed.

While in the accompanying drawings the circuits are arranged to test trunk groups divided into three sections, it is imderstood that the invention is not to be so limited. It will be apparent to ,any one skilled in the art that other sets of relays corresponding;

to the relays 402, 816, and 817 may be provided, thus allowing the test relays 800 to be associated in succession with as many sections as the selected trunk What is claimed is 1. In combination,': a group of trunks divided into sections, a testing device, meansfor connecting said device to the first section of said group of trunks to test the trunks thereof separately and simultanegroup contains.

ously, nleans'for causing disconnection of said device from the trunks of said first section when all said trunks are busy, and means for thereupon causing reconnection of said device to succeeding sections of said trunk group to test for an 1d1e trunk.

2.'In combination a selective switch, a

plurality of groups of trunks, each group" divided into sections, a testing device, means for partially operating and releasing said selective switch to select a particular one of said groups of trunks, means for connecting said device to a first section of said particular group of trunks for making a simultaneous test thereof, means for causing disconnection of said device from said first section when all the trunks therein are busy, and means for thereupon causing said device to be connected in succession to other sections.

of said group of trunks.

3. In combination, a group of trunks comprising a plurality of sections, a testing device, COIIIIGCtlIlg means comprising a pluing device with the first section of said group of trunks, and means for causing disconnection of said device fromsaid first section when all the trunks therein are busy and for associating said device with succeeding sections of said trunk group until an idle trunk is found.

4. In combination, a selective switch, groups of trunks each group being divided into sections, means for (partially operating and releasing sai switch, tothereby mark a particular one of sa1d groups of trunks, a testing. device, a first connecting means, a second connecting means, a third connecting means,

means for actuating said first connecting means to associate said testing device with the first section of said group of trunks, means effective when all the trunks in said first section are busy for releasing said first connecting device and operating said second connecting means to associate said testing device with the second section of said group of trunks, and means effective when all the trunks in said secondsection are busy for releasing said second connecting means and operating said third connecting means to associate said testing device with the third section of said group of trunks.

5. In combination, a selective switch, groups of trunks each group being divided into sections, means for partially operating and releasing said switch to mark a particular one of said groups of trunks, a testing device, variably operable connecting means comprising series of multi-contact relays each relay individual to a corresponding section in said group of trunks, means for actuating said COl'lIlECtiIlg device to associate said testing device with the first section of said group of trunks and means effective when all the trunks in said first section are busy for releasing said connecting device and causing successive operations of the same to thereby associate said testing device with the remaining sections of said group of trunks.

6. In a telephone exchange system, acalling line, groups of trunks, each group divided into sections, selective switches for extending said calling line to a particular one of said groups of trunks, a controlling mechanism for controlling the operation of certain of saidselective switches, testing means in said controlling mechanism, means for associating said testing means with a first section of said group of trunks to make a simultaneous test thereof, and means effective when all the trunks in said first section are busy for releasing said testing means and associating the same with succeeding sections of said group of trunks for the purpose of selecting an idle trunk therein.

7. In a telephone exchange system, a calling line, groups of trunks, each group di-, vided into sections, selective switches for extending said calling line to a particular one of said groups of trunks, a controlling mechanism for controlling the operation of certain of said selective switches, testing means in said controlling mechanism. connecting means comprising series of multicontact relays for associating said testing means with successive sections of said group of trunks, and means effective when all the trunks in one section are busy for releasing said testing and connecting means and for reoperating said connecting means to associate said testing means with succeeding sections of said roup of trunks for the purpose of selecting an idle trunk therein.

-8. In a telephone exchange system, a calling line, groups of trunks, each group divided into sections, selective switches for extending said calling line to a particular one of said groups of trunks, a controlling mechanism for controlling the operation of certain of said selective switches, testing means in said controlling mechanism, connecting means comprising series of multicontact relays certain of said multicontact relays being individual each to a corresponding section of said grou of trunks, and means effective when all t e trunks in a section are busy for reoperating said connecting means and releasing said testing means to thereby associate said testing means with succeeding sections of said group of trunks for the purpose of selecting an idle trunk therein.

9. In a telephone exchange system, a calling line, a private branch exchange, groups of trunks leading to said private branch exchange, each group being divided into sections, selective switches for extending said calling line to said groups of trunks, means for partially operating and releasing one of said switches to thereby mark a particular one of said groups of trunks, a controlling mechanism for controlling the operation of certain of said selective switches, testing means in said controlling mechanism, a first connecting means rendered effective when said particular group of trunks has been marked for associatin said testing means with a first section 0 said particular group of trunks, a second connecting means, and means effective when all the trunks in said first section are busy for releasing said first connecting means and for operating said second connecting means to thereby associate said testing means with a second section of said group of trunks.

10. In a telephone exchange system, a calling line. a selective switch for extending said calling line, groups of trunks arbitrarily located in said selective switch, each group divided into sections, means for partially operating and releasing said selective switch to thereby mark a particular one of said groups of trunks, a controlling mechanism for controlling the operation of said selective switch, testingmeans in said controlling mechanism, variably operable connecting means rendered effective when said particular trunk group has been marked for associating said testing means with a first section of said group of trunks, means effective when all the trunks of said first section are busy for releasin said connecting means and automaticalIy reoperating the same to associate said testing means in succession with other sections of said group of trunks to thereby select an idle one thereof, and means ei'fective when an idle trunk has been found for identifying its location in said selective switch.

11. In a telephone exchange system, a calling line, selective switches for extending said calling line, a private branch exchange, groups of trunks leading to said private branch exchange, each group divided into sections, means for partially operating and releasing one of said switches to mark a particular one of said groups of trunks, a controlling mechanism for controlling the operation of certain of said selective switches, testing means in said controlling mechanism, a variably operable connecting device comprising a plurality of sections, each section individual to a corresponding section of said group of trunks, means effective when said group of trunks has been marked for causing the actuation of said variably operable connecting device to associate a first section of said group of trunks with said testing means, and means efi'ective when all the trunks in said first section are busy for reoperating said connecting means to disconnect said testing means from said first portion of said group of trunks and to associate said testing means with a succeeding section of said group of trunks.

12. In a telephone exchange system, a calling line, selective switches for extending said calling line, a private branch exchange, groups of trunks leading to said private branch exchange, each group of trunksbeing divided into sections and arbitrarily located in one of said selective switches, means for partially operating and releasing said one of said selective switches to mark a particular one of said groups of trunks, a controlling mechanism for controlling the operation of certain of said selective switches, testing means in said controlling mechanism, a variably operable connecting device comprising sections each section individual to a corresponding section of said group of trunks, means effective when said group of trunks has been marked for causing the actuation of, said variably operable device to associate a first section of said group of trunks with said testing means, means effective when all the trunks in said first section are busy for reoperating said connecting device to disconnect said testing means from said first section of said group of trunks and for associating said testing means with succeeding sections of said group of trunks for the purpose of selecting an idle one thereof, and means effective when an idle trunk has been found for identifying its location in said one of said selective switches.

13. In a telephone exchange system, a callof trunks leading to said private branch exchange, each group arranged in sections, seiective switches for extending said calling line to a particular one of said groups of trunks, a controlling mechanism for controlling the operation of certain of said selective switches, testing means in said controlling mechanism, a variably operable connecting device comprising a plurality of multicontact relays, each relay individual to a corresponding section of said group of trunks, means 'for operating one of said multicontact relays to associate said testing means with the corresponding section of said group cf trunks, and means effective when all the trunks of said first section are busy for automatically operating the second one of said multicontact relays to thereby release said first multicontact relay and associate said testing means with a second section of said group of trunks, means eifective when all the trunks in said second'section are busy for operating a succeeding one of said multicontact relays to thereby release said second multicontact relay and to associate said testing means with a succeeding section of said group of trunks.

In witness whereof, I hereunto subscribe my name this 2nd day of July, A. D., 1924.

EDGAR H. CLARK. 

